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324                             CHEMICAL ENGINEERING
Magnetic and electrostatic concentration are applicable to a range of size from about 12-mesh to the finest sands. They are seldom used as primary methods but in conjunction with the concentrations made by employing principle (2).
Unlocking.—Before tests can be made or concentration can begin the ore or other material to be dressed must be unlocked by crushing and comminution. The aim of crushing and comminuting operations should be to make the minimum of the finest material since this is the part most difficult to treat. It is usual to crush to a point where some commercial or waste material will issue on employing concentration operations and this first crushing may be termed preliminary crushing. The unfinished products which arise by concentrating the material which has undergone a" preliminary crushing will need further reduction before being again submitted to the finishing concentrating operations. This recrush may be termed secondary crushing.
Preliminary Crushing.—Crushers, rolls and disc crushers are the principal machines employed in preliminary crushing. Rolls, edge runners, ball, rod and tube mills are employed in secondary crushing. But the general principles of crushing are extensively treated under " Crushing and Grinding" and will not be further referred to here except as necessary for this subject.
Principle of Secondary Reducing Machines.—In preliminary crushing machines the material to be crushed is caught between surfaces or crushing masses inclined to one another and is broken by the direct application of pressure.    The impact machines for coarse crushing work are but little used in concentrating work.   Owing to wear and lost motion it is impossible to extend the principle of coarse crushing into that of fine because the crushing faces could not be kept in pressive contact with the material to be crushed.    The corrugations and irregularities of surface caused by wear will often prevent the crushing surfaces from being brought in intimate contact and fine material will escape through interstices formed this way.   It is consequently necessary in designing secondary crushing machines to have a steel case enclosing the crushing faces or masses the ore being introduced inside of such case and more or less surrounding the crushing faces or masses.   With edge runners such as Chilean and Huntington mills the crushing masses are rotated by power.   In the case of ball and tube mills the balls or pebbles rise by centrifugal force and friction until these forces are overcome by gravity.   They then cascade upon the ore and balls or pebbles.    In the case of stamps weights are raised by power and are then allowed to fall on the material to be crushed.    Crushing in these cases is partly by impact and partly by attrition.   In the case of the edge runners crushing is partly by pressure and partly by attrition.   The difference in crushing principle between those applied in preliminary crushing and secondary crushing are that in the first case crushing is done by direct application of pressure the material being fed in a thin stream so that the greater bulk of the particles are crushed by being directly caught between the crushing faces.   In secondary machines to overcome the impractibility of direct seizure of fragments between crushing faces, crushing where done by pressure or impact, which gives the same effect, is en masse, there being a relatively great number of fragments in the lines of the application of pressure.
In preliminary crushers the route of the material to be crushed from entry to exit point is as direct as possible. In secondary reducing machines the route is long so as to afford the fragments the opportunity to come under the influence of the haphazard